The present invention relates generally to the television art and more particularly to the vertical centering of the video picture on the television screen. Even more particularly, the present invention relates to the phase correction of the vertical drive signal, which controls the vertical flyback in a raster display, to overcome the "real-time" response lag in a television receiver.
In a typical television system, a broadcasting station transmits a television signal which is picked up by a receiver either off the airwaves by means of an antenna or directly from a transmission cable. The receiver then translates this signal for display on the television screen. Typically, video is displayed on the screen via a display beam in a time-coordinate, raster-like fashion. The horizontal and vertical sweeping action of the display beam is normally controlled by circuits within the receiver which are responsive to vertical and horizontal synchronizing information also contained in the transmitted signal.
The horizontal and vertical sync information in the transmitted signal typically takes the form of pulses of defined duration. Also, the horizontal sync pulses and the vertical sync pulses occur during horizontal and vertical blanking intervals, respectively, during which no video information is present in the transmitted signal. Circuits within the television receiver respond to these pulses to cause the sweeping display beam to flyback to the left edge of the screen at the end of each horizontal line and to flyback to the upper left corner of the screen at the end of each vertical field. Thus, in an ideal receiver, the horizontal and vertical flybacks occur during the respective horizontal and vertical blanking intervals when no video information is present to appear on the screen. It may be desireable however to cause the vertical flyback to occur at some fixed, predetermined time other than the vertical blanking interval in order to implement a particular effect or feature. In other words, the phasing of the vertical flyback relative to the blanking period should be made controllable which, in effect, allows for the predetermined control of the vertical picture centering function.
More importantly, however, the circuits within the television receiver normally do not instanteously recognize the vertical sync information. Typically, vertical sync information will have been present for 50 microseconds or longer before the sync circuit reacts to trigger a vertical drival signal to return the video display beam to the upper left hand corner of the screen and begin a new vertical display field. Consequently, the new display field does not begin coincidently with the initiation of the vertical sync pules which define the vertical display field; there is a "real time" lag approximately corresponding to the recognition time of the particular vertical sync detecting circuit employed.
Significantly though, many teletext systems transmit information immediately following the vertical blanking interval. At times this information may be vital teletext page organizing instructions and at other times it may be data to be displayed as part of a complete teletext page or a header at the top of the screen. In such instances it is undesirable that this information be lost when displaying the page. However, because of the "real time" lag in the vertical sync detection and vertical drive circuits, the information contained near the vertical blanking interval will usually be lost from the display entirely, because the picture is vertically offset.
Therefore, it is a principal object of the present invention to provide a means and method for initiating each vertical display field more nearly coincidently with the initiation of the vertical sync information in the transmitted signal.
It is a related object of the present invention to provide a means and method for the control of the vertical centering of a television picture.
It is a further related object of the present invention to provide a means and method for statically correcting the phasing of a television picture display.
It is still a further object of the present invention to modify and utilize existing circuits in a television system providing for a means of exacting the control over the phasing of the display while maintaining compatibility with such existing circuitry.
It is yet a further object of the present invention that it accomplish its objectives at a low cost with a small number of components.
Accordingly, the present invention provides for a correction factor to be introduced into the generation of the vertical drive signal to eliminate, at least partially, the "real time" lag introduced by the vertical sync detection circuits. Relying upon known characteristics of the sync detection circuits and a standard transmitted signal, the preferred embodiment of the present invention changes the preset starting count for the receiver's vertical countdown circuit (described in more detail in U.S. Pat. No. 4,298,890 entitled "Digital Vertical Synchronization System For A Television Receiver"). Utilizing a vertical countdown as presently incorporated in many television receivers avoids undesirable jittering of the picture, which may be introduced otherwise. To compensate for the delay in recognition of the vertical sync pulses, typically about one to one and one-half lines, the preset count of the counter is advanced from "20" to "22" which ultimately causes the vertical drive signal to be generated one horizontal line sooner than normal. To avoid circuit runaway, the "22" preset is enabled only once for each standard signal detected. Thereafter, the countdown circuit generates a vertical drive pulse every 525 counts as it normally would. Thus, this aspect of the present invention shifts the time phase of the vertical drive signals back in time one horizontal line period. Another aspect of the preferred embodiment senses when the system changes from a nonstandard mode, typical of use with a home computer or video recorder, to a standard mode, typical of a normal transmitted signal, to trigger the one-time adjustment of the countdown circuit preset. Yet another aspect of the preferred embodiment provides for the generation of a delayed pulse to be compared in a coincidence detector with the integrated vertical signal derived from the vertical sync pulses. This coincidence detection is used for other purposes such as frequency and phase detection as described in U.S. Pat. No. 4,298,890.